linux-kernel - Re: [PATCH RFC ticketlock] Auto-queued ticketlock

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Date:	Fri, 14 Jun 2013 23:36:45 -0400
From:	Waiman Long <waiman.long@...com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, ????????? <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Valdis Kletnieks <Valdis.Kletnieks@...edu>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <darren@...art.com>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	Silas Boyd-Wickizer <sbw@....edu>
Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock

On 06/14/2013 09:26 PM, Benjamin Herrenschmidt wrote:
> On Fri, 2013-06-14 at 14:17 -0400, Waiman Long wrote:
>> With some minor changes, the current patch can be modified to support
>> debugging lock for 32-bit system. For 64-bit system, we can apply a
>> similar concept for debugging lock with cmpxchg_double. However, for
>> architecture that does not have cmpxchg_double support, it will be out
>> of luck and we probably couldn't support the same feature in debugging
>> mode. It will have to fall back to taking the lock.
> That means only x86_64 and s390 would benefit from it ... I'm sure we can do better :-)
>
> Cheers,
> Ben.

On second thought, using cmpxchg_double may not be such a good idea 
after all as it requires a 16-byte alignment, at least for x86-64. 
Another possible alternative is to integrate the reference count 
directly into the spinlock_t data structure immediately after 
arch_spinlock_t for this special case. If CONFIG_GENERIC_LOCKBREAK is 
not defined, there will be a 4-byte hole that can be used. Otherwise, 
the spinlock_t structure will have an 8 byte size increase. I suppose 
that others won't be too upset for an 8-byte increase in size when 
spinlock debugging is turned on.

Regards,
Longman
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